At present, light water reactor designs depend on the cooling capabilities of water at high pressures and temperatures. However, designs using water as coolant at high pressures have to account for the loss of coolant in the nuclear reactor, by breach in structural integrity. This is due to the increase in operating temperature of the coolant which results in increased boiling of the pressurized coolant, affecting the heat transfer, reactivity parameters and leads to instability issues in the reactor. In order to maintain the coolant state, the pressure needs to be increased which affects the structural integrity and leads to loss of coolant in the reactor. Thus, the management of the reactor core beyond a certain temperature depends on depressurizing and cooling, which currently involves external components and their availability.
Various systems of light water reactor designs are known in the prior art and a few of them are discussed below:
Pre-Pressurized Core Flooding Accumulators:
In this decay heat transport system are connected to primary heat transport system through check valves at pressures lesser than accumulator pressure in case of LOCA (Loss Of Coolant Accident) and inventory is sufficient for 15 minutes (Effective from 6% decay power depending on primary heat transport system pressure). It has a design capacity of 15 minutes. To achieve the said effect, it uses a significant number of other components such as high pressure accumulators, piping, isolation valves, instrumentation, which results in high density of components in the containment.
Core make-up tanks are available that are connected to primary heat transport system through valves to initiate natural circulation based on the design sequence and are effective for 6% decay power; having a design capacity of 3 days. It also uses numerable components such as tank inside the containment, piping, valves, instrumentation, resulting in high density of components in the containment.
Gravity drain tanks: system is connected to reactor primary coolant system through valves, only at low reactor pressure (Effective for 1% decay power depending on the primary heat transport system pressure). It has a design capacity of 3 days and uses components such as tank above the core elevation, piping, valves, instrumentation etc.
Passively Cooled Steam Generator Natural Circulation:
In this system is connected to steam generator by valves, actively or passively based on pressure (Effective for 6% decay power depending on primary heat transport system operation) and has low design capacity of one day, depending on the tank design capacity. It uses high number of components such as heat exchangers, water pool immersing the heat exchangers, piping, instrumentation etc.
Passively cooled Core Isolation condenser system connect to primary heat transport system through valves actively or passively. They are effective for 6% decay heat depending on the primary heat transport system pressure. The design capacity is more than 3 days and has components such as isolation condenser, water pool, piping, instrumentation etc, which leads to high density of the components in the containment.
Prior-art U.S. Pat. No. 4,608,224 A provides nuclear reactor cooled by a liquid metal. It teaches a shut-down heat exchanger means operable during reactor shut-down conditions and for establishing a thermal siphon effect. Further, it teaches a difference in level between reactor core (hot source) and exchangers (cold source), which aids the formation of a thermal siphon within the main reactor vessel, when the external circuits have been emptied into the latter. This feature, facilitates the cooling of the core by the thermal siphon effect.
In view of the above, a number of designs are known which are based on different concepts. Thermo-siphon cooling, heat transport by fins to air and liquid metal coolants are known in different nuclear reactor designs and industrial applications other than water cooled reactors. However, Fast reactors operate at temperatures above 500° C. and use liquid metal coolant compatible at high temperatures while water cooled reactors operate in thermal region of neutron spectrum by the moderating properties of water and operate at temperatures around 300° C., and at its saturation pressures.
All the above, decay heat removal systems existing in different reactor systems remove heat from the primary coolant only and not directly from the core. Decay heat removal in these system designs depend on the availability of primary coolant and integrity of the system to deliver intended functionality. Also, these systems require a large number of components, which increases the density of components in the containment.
Recent accidents in the Fukushima boiling water reactors (BWRs), Japan, established that there is a need for decay heat transport directly from the core in addition to the heat transport from the primary coolant. The severe accidents in water cooled reactors clearly indicate the necessity of core decay heat transport at high temperatures beyond water cooling, for ultimate safety of the core.
Present water cooled reactor designs incorporate saturated water in single or two-phase as their primary heat transport medium and the heat transfer to the main heat sink, is mainly achieved due to fluid to fluid conduction and convection modes of heat transfer away from the core. In the accident scenarios, such as that of Fukushima, the core suffers the absence of intended heat sink and poor heat transfer at high temperatures in addition to other influence vectors. Hence to contain the accident conditions beyond the high pressure water based heat transport, a diverse passive decay heat transport mechanism is needed. Hence, it is desirable to have a system, device and method for water cooled reactors that transports decay heat directly from the fuel to ultimate heat sink.